Depending on what figures you use, the United States is either going to spend as much as $1 trillion — with a T — on the F-35 program over its life or something closer to $379 billion to design and build 2,443 of the fifth-generation fighter. That second number does not include the costs of operating and maintaining the aircraft, which typically comprise 70 percent or more of a weapon systems total cost, so the first number is likely more accurate. But how accurate? The numbers are very difficult to pin down so early in the program. For all the effort the Pentagon, the intelligence agencies and NASA put into it, cost estimating and judging life cycle costs is daunting work.

And, as the CEO of F-35 builder Lockheed Martin told Congress, that trillion-dollar estimate is based on a big change in how the Pentagon estimates the Joint Strike Fighter’s costs.

The original estimate, made in 2001, assumed a 30-year life for the plane. But in 2006 the Pentagon changed that to 50 years, CEO Bob Stevens said. Also, F-35 supporters say the Pentagon estimates are deeply flawed because they rely on historical models — meaning the last similar plane we built, the F-16, and its cousins, the F-15 and the F-18. They argue the F-35’s predictive maintenance software and sensors — which will tell mechanics when a part needs replacing before it fails — should lead to much lower maintenance costs.

On the other hand, the F-35 is a stealthy aircraft. The materials and techniques used to achieve stealth are historically Expensive.

For those who watch these programs closely, one other fact stands out. The costs for these aircraft — as for most U.S. military aircraft — does not include the engines, made by Pratt & Whitney. (General Electric and Rolls Royce have been building the so-called alternate engine for the plane, but Defense Secretary Robert Gates recently killed that program after Tea Party supporters in the House voted against it.) If you want to know how much the whole plane costs, then add something like $100 billion.